Ni-containing steel plate

ABSTRACT

An object of the present invention is to provide an Ni-containing steel plate which is low in cost and has excellent low-temperature toughness. In view of the object, the Ni-containing steel plate of the present invention has a chemical composition containing by mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn: 0.45% to 2.00%, P: 0.020% or less, 5: 0.005% or less, Al: 0.005% to 0.100% Ni: 5.0 to 8.0%, and the balance being Fe and incidental impurities, and has a microstructure containing less than 1.7% by volume fraction of retained austenite when cooled to liquid nitrogen temperature, and having an average grain size of crystal grains surrounded by high-angle grain boundaries with an orientation difference of 15° or more of 5 μm or less by equivalent circle diameter.

TECHNICAL FIELD

The present invention relates to an Ni-containing steel plate withexcellent low-temperature toughness, in particular to a steel platewhich is suitable for use as members such as storage tanks for liquefiednatural gas.

BACKGROUND ART

Conventionally, for members such as overland storage tanks for liquefiednatural gas (hereinafter, referred to as LNG) high Ni-containing steelplates which are excellent in mechanical properties at low temperatureshave been commonly used. In particular, steel plates composed of highNi-containing steel which contains Ni by 9 mass % (hereinafter, referredto as 9% Ni steel) have been commonly used, and they have, actually beenapplied in many cases.

Regarding 9% Ni steel, considerations on various properties such asmechanical properties and weldability have been made. For example, Steeland Iron by Furukimi Osamu, Suzuki Shigeharu, Nakano Yashifumi,69(1982)5, S492 (NPL 1) discloses that low-temperature toughness isimproved by reducing the amount of impurity elements such as P and S.Further, Handbook of Metal, 4^(th) revised edition, edited by The JapanInstitute of Metals and Materials, Maruzen, p 801 (NPL 2) discloses thatlow-temperature toughness is improved by stabilizing retained austenite.However, since Ni is an expensive metal, it is desired to reduce Nicontent.

Techniques for obtaining steel plates which can be made to have an Nicontent smaller than that of 9% Ni steel and has good low temperaturetoughness are disclosed in for example, WO2007/034576 (PTL 1),WO2007/080645 (PTL 2), JP2011-214099A (PTL 3). PTL 1 discloses thatmechanical properties of a steel plate can be improved by predeterminingthe chemical composition of the steel plate, defining the amount, aspectratio, and average equivalent circular diameter of austenite containedin the steel plate, and manufacturing the steel plate with a method tosatisfy such definitions. Further, PTL 2 discloses that toughness of theheat-affected zone of a steel plate can be improved when the steel platehas a predetermined chemical composition and the Fe content obtained byan extraction residue method after a heat-cycle simulation test is morethan a predetermined value. Further, PTL 3 discloses that a brittlecrack-arrest property of steel can be improved when the steel has apredetermined chemical composition, with certain textures developed.

CITATION LIST Patent Literature

PTL 1: WO2007/034576

PTL 2: WO2007/080645

PTL 3: JP2011 -214099A

Non-Patent Literature

NPL 1: Steel and iron by Furukimi Osamu, Suzuki Shigeharu, NakanoYoshifumi, 69(1982)5, S492

NPL 2: Handbook of Metal, 4^(th) revised edition, edited by The JapanInstitute of Metals and Materials, Maruzen, p 800-802

SUMMARY OF INVENTION Technical Problem

However, the techniques disclosed in PTL 1, 2 and 3 do not includedefinitions regarding the amount of austenite at around −165° C. wherethe LNG tanks are actually used, and consideration regardinglow-temperature toughness when the techniques are applied to actualstructures were not made. Further, there were no specific disclosuresregarding the manufacturing method of the steel plates.

The present invention has been developed in view of such situation, andan object thereof is to provide an Ni-containing steel plate which islow in cost and has excellent low-temperature toughness.

Solution to Problem

The inventors of the present invention, as a result of intenseinvestigation for providing an Ni-containing steel plate with excellentlow-temperature toughness, discovered, that by containing C, Si, Mn, P,S, Al, and Ni as essential elements of a steel, and setting the amountof retained austenite contained in the steel after performing sub-zerotreatment were cooling is performed until reaching liquid nitrogentemperature to be less than 1.7%, and setting the average grain size ofcrystal grains surrounded by high-angle grain boundaries with anorientation difference of 15° or more to 5 μm or less by equivalentcircle diameter, excellent low-temperature toughness can be achievedeven when the Ni content is reduced compared to conventional 9% Nisteel.

If the Ni content in steel is reduced to be smaller than that of 9% NiSteel, even if retained austenite is stable at room temperature, it willbe unstable at −165° C. where LNG tanks are used. Further, it isconsidered that toughness decreases when retained austenite exists at−165° C., because the retained austenite is transformed into martensitephase due to deformation induced transformation, at the tip of a crackformed in the steel material when the LNG tank fractures. Under thesituation, by reducing the amount of retained austenite remaining aftersub-zero treatment corresponding to −165° C. where LNG tanks are used,and forming a fine microstructure as described above, it is assumed thatlow-temperature toughness can be improved even if the Ni content insteel is reduced to be smaller than that of conventional 9% Ni steel.

The present invention is based on the above discoveries and it providesthe following (1) to (4).

(1) An Ni-containing steel plate having, a chemical compositioncontaining by mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn: 0.45% to2.00%, P: 0.020% or less, S: 0.005% or less, Al: 0.005% to 0.100%, Ni:5.0% to 8.0%, and the balance being Fe and incidental impurities,wherein

the steel plate has a microstructure containing less than 1.7% by volumefraction of retained austenite when cooled to liquid nitrogentemperature, and having an average grain size of crystal grainssurrounded by high-angle grain boundaries with an orientation differenceof 15° or more of 5 μm or less by equivalent circle diameter.

(2) The Ni-containing steel plate according to aspect (1), wherein thechemical composition further contains by mass % at least one elementselected from Cr: 1.00% or less and Mo: 1.000% or less.

(3) The Ni-containing steel plate according to aspect (1) or (2),wherein the chemical composition further contains by mass % at least oneelement selected from Cu: 1.00% or less, V: 0.100% or less, Nb: 0.100%or less, Ti: 0.100% or less, and B: 0.0030% or less.

(4) The Ni-containing steel plate according to any one of aspects (1) to(3), wherein the chemical composition further contains by mass % atleast one element selected from Ca: 0.0050% or less and REM: 0.0050% orless.

Advantageous Effect of Invention

According to the present invention, an Ni-containing steel platecontaining less Ni content compared to 9% Ni steel but havinglow-temperature toughness equivalent to that of 9% Ni steel can beeasily manufactured, and an industrially remarkable effect is provided.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the Ni-containing steel plate according to the presentinvention will be explained in detail and separately based on chemicalcomposition, microstructure, and manufacturing method.

Unless otherwise specified, the indication of “%” regarding compositionshall stand for “mass %”.

(1) Chemical Composition

First, the chemical composition will be described.

C: 0.01% to 0.15%

C is an important element for solid solution strengthening of steel. IfC content is less than 0.01%, sufficient strength cannot be obtained. Onthe other hand, adding C in an amount exceeding 0.15% would causedeterioration of weldability and workability. Therefore, C content isset to be in the range of 0.01% to 0.15%. Preferably, the range is from0.03% to 0.10%.

Si: 0.02% to 0.20%

Si is an effective element as a deoxidizer in molten steel and aneffective element for solid solution strengthening. If Si content isless than 0.02%, deoxidizing effect cannot be sufficiently obtained. Onthe other hand, adding Si in an amount exceeding 0.20% would causeproblems such as reduction in ductility and toughness, and an increaseof inclusions. Therefore, Si content is set to be in the range of 0.02%to 0.20% and preferably in the range of 0.03% to 0.10%.

Mn: 0.45% to 2.00%

Mn is an effective element from the viewpoint of ensuring quenchhardenability and enhancing strength. If Mn content is less than 0.45%,the effect thereof cannot be sufficiently obtained. On the other hand,adding Mn in an amount exceeding 2.00% would cause deterioration ofweldability. Therefore, Mn content is set to be in the range of 0.45% to2.00%, and preferably in the range of 0.55% to 1.00%.

P: 0.020% or less

Although high P content in steel leads to deterioration of lowtemperature toughness, the content thereof of 0.020% or less would beacceptable. Therefore, the upper limit of P content is set to be 0.020%.

S: 0.005% or less

High S content in steel causes precipitation as MnS, and this, as aninclusion, becomes the fracture generation origin of high tensilestrength steel and leads to deterioration of toughness. However, if thecontent thereof is 0.005% or less, it would cause no problem. Therefore,the upper limit of S content is set to be 0.005%.

Al: 0.005% to 0.100%

Al is an effective element as a deoxidizer in molten steel and aneffective element for improving low-temperature toughness. If Al contentis less than 0.005%, these effects cannot be sufficiently obtained. Onthe other hand, if the content thereof exceeds 0.100%, weldability willdecrease. Therefore, Al content is set to be in the range of 0.005% to0.100%, and preferably in the range of 0.020% to 0.050%.

Ni: 5.0 to 8.0%

Ni is an important element for the present invention, and it is anelement that enhances quench hardenability and improves toughness offerrite matrix. If Ni content is less than 5.0%, these effects cannot besufficiently exhibited. On the other hand, if the content thereofexceeds 8.0%, costs will increase. Therefore, Ni content is set to be ina range of 5.0% to 8.0%. In addition, from the viewpoint of furtherreducing costs, it is desirable for Ni content to be in the range of5.0% to 7.5%.

In addition to the above basic chemical compositions, it is possible tocontain at least one element selected from Cr and Mo, as a first groupof selected components, if necessary, in the following ranges.

Cr: 1.00% or less

Cr enhances quench hardenability and provides an effect of improvinglow-temperature toughness by refining martensite phase. However, if thecontent thereof exceeds 1.00%, it would cause deterioration ofweldability and an increase in manufacturing costs. Therefore, whencontaining Cr, the content thereof is set to be in the range of 1.00% orless. In order to effectively exhibit the above effect, it is preferablefor the Cr content to be 0.05% or more, and more preferably in the rangeof 0.10% to 0.75%.

Mo: 1.000% or less

Mo enhances quench hardenability and provides an effect of improvinglow-temperature toughness by refining martensite phase. However, if thecontent thereof exceeds 1.000%, it would cause deterioration ofweldability and an increase in manufacturing costs. Therefore, whencontaining Mo, the content thereof is set to be in the range of 1.000%or less. In order to effectively exhibit the above effects, it ispreferable for the content thereof to be 0.005% or more, and morepreferably in the range of 0.010% to 0.500%.

Further, in the present invention, it is possible to contain at leastone element selected from Cu, V, Nb, Ti, and B as a second group ofselected components, if necessary, in the following ranges.

Cu: 1.00% or less

Cu is an element that enhances quench hardenability. However, if thecontent thereof exceeds 1.00%, it would cause reduction of hotworkability and an increase in costs. Therefore, when containing Cu, thecontent thereof is set to be in the range of 1.00% or less. In order toeffectively exhibit the above effect, it is preferable for the contentthereof to be 0.05% or more.

V: 0.100% or less

V is an element that precipitates as carbonitride, has an effect ofrefining microstructures, and is useful for improving toughness.However, if the content thereof exceeds 0.100% it would causedeterioration of weldability. Therefore, when containing V, the contentthereof is set to be in the range of 0.100% or less. In order toeffectively exhibit the above effects, it is preferable for the contentthereof to be 0.005% or more.

Nb: 0.100% or less

Nb is an element that precipitates as carbonitride, has an effect ofrefining microstructures, and is useful for improving toughness.However, if the content thereof exceeds 0.100%, it would causedeterioration of weldability. Therefore, when containing Nb, the contentthereof is set to he in the range of 0.100% or less. In order toeffectively exhibit the above effects, it is preferable for the contentthereof to be 0.005% or more.

Ti: 0.100% or less

Ti has an effect of improving toughness by fixing solute N, which isharmful to toughness, as TiN. However, if the content thereof exceeds0.100%, it would cause precipitation of a coarse carbonitride, anddeteriorate toughness. Therefore, when containing Ti, the contentthereof is set to be in the range of 0.100% or less. In order toeffectively exhibit the above effect, it is preferable for the contentthereof to be 0.005% or more, and more preferably in the range of 0.010%to 0.050%.

B: 0.0030% or less

B is an element that enhances quench hardenability when added to steelby a small amount. However, if the content thereof exceeds 0.0030%, itwould cause deterioration of toughness. Therefore, when containing B,the content thereof is set to be in the range of 0.0030% or less. Inorder to effectively exhibit the above effect, it is preferable for thecontent thereof to be 0.0003% or more.

Further, in the present invention, it is possible to contain at leastone element selected from Ca and REM as a third group of selectedcomponents, if necessary, in the following ranges.

Ca: 0.0050% or less

Ca is an element that fixes S and inhibits generation of MnS whichbecomes the cause of reduction in toughness. However, if the contentthereof exceeds 0.0050%, it would cause an increase in the amount ofinclusions existing in steel and lead to deterioration of toughnessrather than providing the above effect. Therefore, when containing Ca,the content thereof is set to be in the range of 0.0050% or less. Inorder to effectively exhibit the above effect, it is preferable for thecontent thereof to be 0.0005% or more.

REM: 0.0050% or less

REM (Rare Earth Metal) is an element that fixes S and inhibitsgeneration of MnS which becomes the cause of reduction in toughness.However, if the content thereof exceeds 0.0050%, it would cause anincrease in the amount of inclusions existing in steel and lead todeterioration of toughness rather than providing the above effect.Therefore, when containing REM, the content thereof is set to be in therange of 0.0050% or less. In order to effectively exhibit the aboveeffect, it is preferable for the content thereof to be 0.0005% or more.

The balance other than the components described above includes Fe andincidental impurities.

(2) Microstructure

Next, the microstructure will be described.

The Ni-containing steel plate of the present invention has the abovechemical composition, and also has a microstructure containing less than1.7% of retained austenite when cooled to liquid nitrogen temperature,and having an average grain size of crystal grains surrounded byhigh-angle grain boundaries with an orientation difference of 15° ormore of 5 μm or less by equivalent circle diameter.

Since the steel plate of the present invention is used mainly in storagetanks for LNG, the microstructure at −165° C. where LNG tanks are usedis important. Therefore, the microstructure after sub-zero treatmentwhere the steel plate is held at liquid nitrogen temperature, isdefined. If the amount of retained austenite remaining after sub-zerotreatment is 1.7% or more by volume fraction, sufficient low-temperaturetoughness cannot be obtained. Some reports have been made that retainedaustenite improves low temperature toughness. However, for theNi-containing steel plate of the present invention, retained austenitehas a harmful effect on toughness. It is considered that this is due tothe fact that, since in Ni-containing steel plate of the presentinvention, the Ni content is smaller than the Ni content in conventional9% Ni steel, even if retained austenite exists at −165° C., it isunstable, and if the steel structure undergoes plastic deformation atthe tip of a crack, the retained austenite transforms into martensite byplasticity-induced martensite phase transformation. Therefore, theamount of retained austenite when the steel plate is cooled to liquidnitrogen temperature is set to be less than 1.7% by volume fraction.This amount is preferably 1.0% or less, and more preferably 0.5% orless.

Further, if the average grain size of crystal grains surrounded byhigh-angle grain boundaries with an orientation difference of 15° ormore exceeds 5 μm by equivalent circle diameter, sufficientlow-temperature toughness cannot be obtained. Therefore, the averagegrain size of crystal grains surrounded by high-angle grain boundarieswith an orientation difference of 15° or more is set to be 5 μm or lessby equivalent circle diameter, and preferably 3 μm or less by equivalentcircle diameter.

(3) Manufacturing Condition

Next, a preferable manufacturing condition for manufacturing the steelplate of the present invention having the above described chemicalcomposition and the above microstructure will be described. Thefollowing manufacturing condition is merely an example of a conditionfor manufacturing the Ni-containing steel plate of the presentinvention, and as long as the Ni-containing steel plate of the presentinvention can be obtained, manufacturing condition for the presentinvention is not limited to the following manufacturing condition.

In the present invention, it is preferable to heat a slab or a steelbillet having the above described chemical composition at a temperaturerange of 900° C. to 1100° C. for 10 hours or less, and then to subjectit to hot rolling at a temperature range of 870° C. or lower so that thecumulative rolling reduction ratio is 40% or more and 70% or less andthe finisher delivery temperature is between 700° C. and 820° C., andthen to subject the obtained hot rolled steel plate to direct quenchingtreatment where quenching is immediately performed until reaching atemperature of 200° C. or lower at a cooling rate of 5° C./s or more,and then to heat the steel plate to a temperature range of 500° C. to650° C. at a heating rate of 0.05° C./s to 1.0° C./s, and then tosubject the steel plate to tempering by holding the temperature at thesame temperature range for 10 minutes or more and 60 minutes or less.

Heating Temperature: 900° C. to 1100° C., Heating duration: 10 hours orless

In a case where the heating temperature is lower than 900° C., coarseAlN which precipitates during the stage of casting of the steel slabdoes not dissolve, and toughness decreases. Further, the followingrolling conditions cannot be substantially satisfied. If the heatingtemperature exceeds 1100° C, austenite becomes coarse grains andtoughness will decrease. If the heating duration exceeds 10 hours,austenite grains become coarse and toughness decreases. Therefore, theheating temperature is set to be between 900° C. and 1100° C., and theheating duration is 10 hours or less.

Rolling Reduction Ratio: Cumulative Rolling Reduction Ratio of 40% ormore and 70% or less at 870° C. or lower

If the cumulative rolling reduction ratio in the non-recrystallizedregion of austenite at 870° C. or lower is less than 40%, refinement ofmartensite phase will not be sufficient, and toughness decreases. On theother hand, in a case where the cumulative rolling reduction ratioexceeds 70%, it is difficult to substantially perform rolling at thefollowing finisher delivery temperature. Therefore, the rollingreduction ratio is set to be 40% or more and 70% or less at 870° C. orlower.

Finisher delivery temperature: 700° C. to 820° C.

If the finisher delivery temperature is lower than 700° C., it resultsin α-γ dual phase rolling so that bainite phase forms, and therefore adesired strength cannot be satisfied. On the other hand, if the finisherdelivery temperature exceeds 820° C., it becomes substantially difficultto perform sufficient rolling reduction in the non-recrystallized regionof austenite, a fine microstructure cannot be obtained, and toughnessdecreases. Therefore, the finisher delivery temperature is set to be inthe range of 700° C. to 820° C.

Cooling (Direct Quenching): Start immediately after rolling

Cooling (direct quenching) is started immediately after rolling isfinished. If cooling is not immediately started, bainite phase willgenerate, and. therefore a desired strength cannot be satisfied.Therefore, cooling is started immediately after rolling is finished.Here, “immediately” refers to a point in time within 120 seconds afterthe completion of rolling.

Cooling Rate; 5° C./s or more

In a case where the cooling rate is less than 5° C./s, transformation tomartensite phase will not occur, and a desirable strength and toughnesscannot be obtained. Therefore, the cooling rate is set to be 5° C./s ormore. Preferably, the cooling rate is 10° C./s or more.

Cooling Stop Temperature: 200° C. or lower

In a case where the cooling stop temperature exceeds 200° C.,transformation to martensite phase will not Occur uniformly in the steelplate, and a desirable strength and toughness cannot be obtained.Therefore, the cooling stop temperature is set to be 200° C. or lower.

Tempering Heating Rate: 0.05° C./s to 1.0° C./s

In a case where the tempering heating rate is less than 0.05° C./s, theprecipitated carbide would become coarse, and toughness will decrease.On the other hand, in order to perform rapid short time beating wherethe tempering heating rate exceeds 1.0° C./s, induction heatingfacilities and the like will be required, and costs will increase.Therefore, the tempering heating rate is set to be in the range of 0.05°C./s to 1.0° C./s.

Tempering temperature: 500° C. to 650° C.

In a case where the tempering temperature is lower than 500° C.,toughness improving effect caused by precipitation of fine carbides suchas cementite cannot, be sufficiently obtained. On the other hand, in acase were the tempering temperature exceeds 650° C., coarse carbideprecipitates, and toughness decreases. Therefore, the temperingtemperature is set to be in the range of 500° C. to 650° C.

Tempering Holding Time: 10 minutes or more and 60 minutes or less

In a case where the tempering holding time is less than 10 minutes,toughness improving effect caused by precipitation of fine carbides suchas cementite cannot be sufficiently obtained. On the other hand, in acase where the tempering holding time exceeds 60 minutes, toughness willdecrease due to reasons such as precipitation of a coarse carbide.Further, manufacturing costs will increase. Therefore, the temperingholding time is set to be 10 minutes or more and 60 minutes or less.Cooling, after tempering may be performed by either water cooling or aircooling. However, if the cooling rate is too fast, the temperaturedifference between the surface and the inside of the steel plate becomeslarge and causes formation of strains inside the steel plate and lowtemperature toughness decreases. Therefore, the cooling rate ispreferably 5° C./s or less.

In the aforementioned manufacturing condition, after direct quenching,dual phase heat treatment where the steel plate is heated to atemperature range from 650° C. to 800° C. at a heating rate of 0.1° C./sto 1.5° C./s, held at the same temperature range for 10 minutes or moreand 60 minutes or less, and then subjected to quenching until reaching atemperature of 200° C. or lower at a cooling rate of 5° C./s or more,may be performed.

Dual Phase Heat Treatment Heating Rate: 0.1° C./s to 1.5° C./s

By performing dual phase heat treatment, part of the microstructuretransforms into austenite, and as crystal grains become fine, temperingproceeds and thereby improves toughness. However, in a case where thedual phase heat treatment heating rate is less than 0.1° C./s, austenitegrains become coarse and toughness decreases. Further, since themicrostructure generated after cooling also becomes coarse, toughnessdecreases. On the other hand, in a case where the heating rate exceeds1.5° C./s, induction heating facilities and the like are required, andcosts increase. Therefore, the dual phase heat treatment heating rate isset to be in the range of 0.1° C./s to 1.5° C./s.

Dual Phase Heat Treatment Temperature: 650° C. to 800° C.

In a case where the dual phase heat treatment temperature is lower than650° C., sufficient austenite reverse transformation does not occur, andrefining effect of the microstructure cannot be obtained, and thereforea toughness improving effect cannot be obtained. Further, since theamount of austenite reverse transformation is small, C easilyconcentrates in austenite, and retained austenite increases. On theother hand, if the dual phase heat treatment temperature exceeds 800°C., reverse transformation austenite becomes coarse and toughnessdecreases. Further, since the microstructure after cooling becomescoarse, toughness decreases. Further, manufacturing costs increase.Therefore, the dual phase heat treatment temperature is set to be in therange of 650° C. to 800° C. In a case where the dual phase heattreatment temperature is high, the amount of reverse transformationaustenite increases and the amount of concentration of C in reversetransformation austenite decreases compared to a case where the dualphase heat treatment temperature is low, and therefore the amount ofmartensite transformation caused h cooling after dual phase heattreatment increases, and the amount of retained austenite decreases.Therefore, the dual phase heat treatment temperature is preferably inthe range of 720° C. to 780° C.

Dual Phase Heat Treatment Holding Time: 10 minutes or more and 60minutes or less

If the dual phase heat treatment holding time is less than 10 minutes,sufficient austenite reverse transformation does not occur and toughnessimproving effect caused by refinement of the microstructure cannot besufficiently obtained. On the other hand, in a case where the dual phaseheat treatment holding time exceeds 60 minutes, austenite grains becomecoarse and toughness decreases. Further, since the microstructuregenerated after cooling also becomes coarse, toughness decreases. SinceC concentrates in austenite, retained austenite increases. Manufacturingcosts increase as well. Therefore, the dual phase heat treatment holdingtime is set to be 10 minutes or more and 60 minutes or less.

Cooling Rate after Dual Phase Heat Treatment: 5° C./s or more

In a case where the cooling rate is less than 5° C./s, transformationfrom austenite to martensite phase will not occur, and a desirablestrength and toughness cannot be obtained. Further, if the cooling rateis slow, the amount of solute C in ferrite decreases as the temperatureis lowered, and therefore C moves to austenite from the ferritesurrounding the reverse transformed austenite, and C concentrates in theaustenite and the austenite tends to remain as retained austenite.Therefore, the cooling rate is set to be 5° C./s or more. Preferably,the cooling rate is 10° C./s or more.

Cooling Stop Temperature after Dual Phase Heat Treatment: 200° C. orlower

In a case where the cooling stop temperature exceeds 200° C.,transformation to martensite phase will not occur uniformly in the steelplate, and a desirable strength and toughness cannot be obtained.Further, C concentrates in austenite and tends to remain as retainedaustenite. Therefore, the cooling stop temperature is set to be 200° C.or lower.

After performing the dual phase heat treatment and cooling, untilreaching 200° C. or lower, tempering is conducted in the mannerpreviously described. That is, the steel is heated to a temperaturerange of 500° C. to 650° C. at a heating rate of 0.05° C./s to 1.0°C./s, and then subjected to tempering by holding the temperature at thesame temperature range for 10 minutes or more and 60 minutes or less.

EXAMPLES

Next, Examples of the present invention will be described.

Molten steels with the chemical compositions shown in table I wereobtained by steelmaking in a vacuum melting, furnace and made intosmall-sized steel ingots (150 kg). These steels were heated in theconditions shown in table 2, subjected to hot rolling until reaching aplate thickness of 7 mm to 50 mm, and then subjected to quenching justafter the rolling. Some of the steel plates were then subjected totempering treatment. Regarding the rest of the steel plates, afterquenching, they were subjected to dual phase heat treatment and then totempering treatment. The obtained steel plates were each subjected to atensile test, a Charpy impact test, a measurement of austenite volumefraction, and a measurement of grain size of crystal grains surroundedby high-angle grain boundaries with an orientation difference of 15° ormore, in the manner described below.

TABLE 1 Steel Chemical Composition (mass %) No. C Si Mn P S Al Ni Cr MoCu V Nb Ti B Ca REM Remarks A 0.06 0.06 1.21 0.005 0.0011 0.035 5.7 — —— — — — — — — Inventive Example B 0.07 0.09 0.95 0.010 0.0009 0.033 7.2— — — — — — — — — Inventive Example C 0.05 0.04 0.67 0.003 0.0012 0.0297.8 — — 0.12 — — — — — — Inventive Example D 0.09 0.03 1.06 0.009 0.00100.028 6.9 0.12 — — 0.043 — — — 0.0023 — Inventive Example E 0.03 0.050.88 0.004 0.0012 0.033 7.4 0.72 — — — — — — — — Inventive Example F0.02 0.06 1.36 0.008 0.0011 0.036 7.6 — 0.03 — — — — — — — InventiveExample G 0.05 0.08 0.63 0.006 0.0008 0.024 6.8 — 0.41 — — 0.014 — — — —Inventive Example H 0.04 0.07 0.97 0.011 0.0008 0.031 7.3 — — 0.23 — —0.015 0.0012 — 0.0018 Inventive Example I 0.06 0.05 1.02 0.005 0.00090.030 4.9 — — — — — — — — — Comparative Example The underlined valuesare outside the scope of the invention.

Tensile Test

From each steel plate, at a position of a half the plate thickness, andin the roiling direction, a tensile test specimen having a parallelportion length of 30 mm, GL of 24 mm, a parallel portion diameter of 6φwas collected and subjected to a tensile test at room temperature. Fromthe obtained stress-strain curve, tensile strength (TS) and yieldstrength (YS) were calculated. TS of 690 MPa or more and YS of 590 MPaor more are each considered as excellent TS and YS.

[Charpy Impact Test]

From each steel plate, at a position of a half the plate thickness, andin a direction orthogonal to the rolling direction, V-notch testspecimens were collected in accordance with JIS Z2202 (1998) standard,and subjected to a Charpy impact test with 3 specimens per eachtemperature for each steel plate in accordance with JIS Z2242 (1998)standard, and absorbed energy at −196° C. was measured to evaluate basematerial toughness. Steel plates with an average value of absorbedenergy (vE.₁₉₆) of 3 specimens of 150 J or more are considered as havingexcellent base material toughness.

[Austenite Volume Fraction]

Samples collected from each steel plate at a position of a half theplate thickness in a direction orthogonal to the rolling direction weresubjected to sub-zero treatment for 10 minutes in liquid nitrogen, andthen the austenite volume fraction was measured by X-ray diffraction.

[Measurement of Grain Size of Crystal Grains]

Samples collected from each steel plate at a position of a half theplate thickness in a direction orthogonal to the rolling direction werepolished and mirror finished, and subjected to EBSP analysis. Among theobtained data, a high-angle grain boundary where the orientationdifference between two crystal grains contacting the grain boundary is15° or more was selected and the average grain size by equivalent circlediameter of the region surrounded by the high-angle grain boundary wasobtained.

The obtained results are shown in Table 2.

As shown in table 2, it has been confirmed that the inventive exampleshave excellent low-temperature toughness whereas the comparativeexamples outside the scope of the present invention have reducedlow-temperature toughness.

TABLE 2 Temp. Dual Dual Cooling Rolling Start of Cool- Phase Heat DualPhase Heat Rate Plate Heat- Reduc- Finisher of Starting Cool- ingTreatment Phase Heat Treatment after Dual Steel Thick- ing tion DeliveryCool- Cool- ing Stop Heating Treament Holding Phase Heat Plate Steelness Temp. Ratio* Temp. ing** ing Rate Temp. Rate Temp. Time TreatmentNo. No. (mm) (° C.) (%) (° C.) (s) (° C.) (° C./s) (° C.) (° C./s) (°C.) (min) (° C./s) 1 A 25 1050 50 780 34 760 25 150 0.34 750 20 22 2 A25 1050 60 750 35 730 25 150 0.37 640 20 22 3 A 25 1050 55 730 38 710 25150 — — — — 4 B 25 1050  0 900 30 880 25 150 0.33 740 30 22 5 B 25 105055 800 34 780 25 150 0.33 740 20 22 6 B 25 1050 55 800 34 780 25 1500.33 630 30 22 7 B 25 1050 55 800 34 780 25 150 0.33 740 120  22 8 B 251050 55 800 34 780 25 150 0.33 740 30  2 9 B 25 1050 55 800 34 780 25150 0.33 700 30 20 10 B 25 1000 60 750 36 730 25 150 0.31 730 20 22 11 B25 1000 55 740 37 720 25 150 0.29 720 15 22 12 B 25 1000 50 800 33 78025 100 — — — — 13 B 25 1050 60 740 37 720 25 150 — — — — 14 B 25 1000 55910 39 870 25 100 — — — — 15 B 7 1000 65 800 18 780 40 100 0.64 740 1037 16 B 50 1050 50 780 60 760 12 150 0.19 720 20  9 17 C 25 1050 60 78034 760 25 150 0.31 730 20 22 18 C 25 1050 55 800 33 780 25 150 — — — —19 C 25 1050 55 810 32 790 25 150 — — — — 20 C 50 1050 60 800 32 780 12100 0.20 720 20  9 21 D 25 1050 65 750 36 730 25 150 0.35 780 50 22 22 E25 1200 50 800 33 780 25 100 0.34 750 20 22 23 E 25  950 55 790 33 77025 150 0.33 740 20 22 24 F 25 1050 60 770 35 750 25 150 0.31 730 30 2225 F 25 1050 30 800 33 780 25 150 0.19 720 20 22 26 G 25 1050 65 730 37710 25 100 0.18 730 20 22 27 H 25 1050 55 750 36 730 25 100 0.29 700 2022 28 H 25 1050 60 780 34 760 25 150 — — — — 29 I 25 1050 60 800 33 78025 150 0.34 760 20 22 Cooling Ave. Grain Stop Temp. Size by after DualTempering Tempering Cooling Austenite Equivalent Steel Phase HeatHeating Tempering Holding Rate after Volume Circle Plate Treatment RateTemp. Time Tempering TS YS vE-196 Fraction Diameter No. (° C.) (° C.) (°C.) (min) (° C./s) (MPa) (MPa) (J) (%) (μm) Remarks 1 100 0.18 570 200.4 698 642 225 0.2 2.3 Inventive Example 2 150 0.19 580 20 0.4 711 382121 2.0 5.6 Comparative Example 3 — 0.17 560 15 0.4 701 650 156 0.3 3.9Inventive Example 4  75 0.18 570 15 0.4 722 699 120 0.2 5.8 ComparativeExample 5 125 0.19 580 20 0.4 740 715 235 0.2 1.9 Inventive Example 6125 0.19 580 20 0.4 810 785 56 2.6 3.6 Comparative Example 7 125 0.19580 20 0.4 822 765 98 1.8 5.1 Comparative Example 8 100 0.19 580 20 0.4752 722 110 1.8 3.2 Comparative Example 9 350 0.19 580 20 0.4 720 735103 1.9 2.5 Comparative Example 10 100 0.19 580 25 0.4 731 685 220 0.31.4 Inventive Example 11 150 0.19 580 20 0.4 705 615 245 0.3 1.1Inventive Example 12 — 0.18 570 40 0.4 715 682 160 0.1 4.3 InventiveExample 13 — 0.20 590 15 0.4 735 719 152 0.1 4.0 Inventive Example 14100 0.19 580 20 0.4 730 705 116 0.3 7.3 Comparative Example 15 150 0.53590 20 1.2 745 719 225 0.2 1.3 Inventive Example 16 100 0.13 600 15 0.2721 674 167 1.4 2.5 Inventive Example 17 100 0.19 580 20 0.4 749 713 2340.2 1.4 Inventive Example 18 — 0.2 590 30 0.4 720 687 174 0.3 4.1Inventive Example 19 — 0.14 520 20 0.4 736 701 151 0.1 4.6 InventiveExample 20 100 0.11 560 15 0.2 732 699 173 1.1 2.0 Inventive Example 21150 0.19 580 20 0.4 726 694 219 0.1 2.0 Inventive Example 22  75 0.17560 15 0.4 713 689 88 0.5 6.7 Comparative Example 23 100 0.18 570 20 0.4720 692 239 0.2 1.2 Inventive Example 24 100 0.2 590 20 0.4 721 675 2150.1 1.5 Inventive Example 25 100 0.19 580 30 0.4 712 653 102 0.3 5.7Comparative Example 26 150 0.13 600 25 0.4 716 665 258 0.2 1.0 InventiveExample 27 150 0.19 580 20 0.4 721 653 182 1.2 1.4 Inventive Example 28— 0.23 630 20 0.4 702 643 176 0.2 4.1 Inventive Example 29 100 0.16 54020 0.4 675 621 76 0.1 1.9 Comparative Example The underlined values areoutside the scope of the invention. *Cumulative rolling reduction ratioat 870° C. or lower **Time from when finishing rolling is completed towhen cooling is started

1. An Ni-containing steel plate having a chemical composition containingby mass % C: 0.01% to 0.15%, Si: 0.02% to 0.20%, Mn: 0.45% to 2.00%, P:0.020% or less, S: 0.005% or less, Al: 0.005% to 0.100%, Ni: 5.0% to8.0%, and the balance being Fe and incidental impurities, wherein thesteel plate has a microstructure containing less than 1.7% by volumefraction of retained austenite when cooled to liquid nitrogentemperature, and having an average grain size of crystal grainssurrounded by high-angle grain boundaries with an orientation differenceof 15° or more of 5 μm or less by equivalent circle diameter.
 2. TheNi-containing steel plate according to claim 1, wherein the chemicalcomposition further contains by mass % at least one element selectedfrom Cr: 1.00% or less and Mo: 1.000% or less.
 3. The Ni-containingsteel plate according to claim 1, wherein the chemical compositionfurther contains by mass % at least one element selected from Cu: 1.00%or less, V: 0.100% or less, Nb: 0.100% or less, Ti: 0.100% or less, andB: 0.0030% or less.
 4. The Ni-containing steel plate according to claim1, wherein the chemical composition further contains by mass % at leastone element selected from Ca: 0.0050% or less and REM: 0.0050% or less.5. The Ni-containing steel plate according to claim 2, wherein thechemical composition further contains by mass % at least one elementselected from Cu: 1.00% or less, V: 0.100% or less, Nb: 0.100% or less,Ti: 0.100% or less, and B: 0.0030% or less.
 6. The Ni-containing steelplate according to claim 2, wherein the chemical composition furthercontains by mass % at least one element selected from Ca: 0.0050% orless and REM: 0.0050% or less.
 7. The Ni-containing steel plateaccording to claim 3, wherein the chemical composition further containsby mass % at least one element selected from Ca: 0.0050% or less andREM: 0.0050% or less.
 8. The Ni-containing steel plate according toclaim 5, wherein the chemical composition further contains by mass % atleast one element selected from Ca: 0.0050% or less and REM: 0.0050% orless.